Effects of grape pomace on ruminal and rectal microbiota and urinary nitrogen emissions in Angus bulls

The primary components of grape pomace (GP) include grape seeds, skins, and stalks, constituting a by-product of the wine industry. This study aimed to examine the impact of grape pomace on the growth performance and nitrogen utilization efficiency of Angus cattle through rumen and rectal microorganisms. Thirty Angus bulls were allocated to three dietary treatments according to a completely randomized design: 0% (G0), 10% (G10), and 20% (G20) dry matter corn silage replacement to dried GP as dry matter. The results showed that the average daily gain (ADG) of G0 and G10 was significantly higher than that of G20 (P < 0.05) and the feed conversion rate (the ratio of DMI and ADG) was significantly lower than that of G20 (P < 0.05); apparent digestibility of ether extract was significantly lower in the G20 than in G0 group and G10 group (P < 0.05); urinary nitrogen decreased linearly with the addition of GP (linear, P < 0.05) and acetate to propionate ratio of G20 group was significantly higher than in the G0 group and G10 group. GP significantly affects the alpha diversity of rectal bacteria, which Chao1 and Shannon index was significantly lower in the G20 group (P < 0.05). In the rumen, succinogenes in the G20 group and Succiniclasticum in the G10 group were the signature genera. In addition, microbial community complexity and stability were better analyzed by interaction network diagrams for G10 group. In the rectum, the abundance of Lachnospiraceae was significantly higher and the abundance of UCG_004 was significantly lower in the G20 group.Overall, based on this study, it can be concluded that adding to 10% GP in Angus bulls rations had no negative effects on growth performance and apparent digestibility, linearly reduced emissions of urinary nitrogen and better microbial community complexity and stability by interaction network diagrams. These findings offer valuable insights into the potential benefits of incorporating GP into the diet of ruminants.